Percussion keyer circuit

ABSTRACT

A tone generator is coupled to an audio reproducing circuit through a variable threshold gate circuit which operates in response to the actuation of a musical instrument key. Means are provided to apply a threshold voltage signal to the threshold gate circuit to initiate conduction of the gate to allow translation of the signals from the tone generator to the audio reproducing means. A current control device has its load electrodes connected between the key contactor and the input of the variable threshold gate circuit to apply an initial pulse thereto and means are provided to automatically terminate the application of such pulse whether said key contactor remains closed or is opened thereafter to simulate the percussive sound of a piano from an electronic organ.

United States Patent Inventor James S. Southard Bristol, Ind.

Appl. No. 22,441

Filed Mar. 25, 1970 Patented Nov. 2, 1971 Assignee C. G. Conn Ltd.

Elkhart, Ind.

PERCUSSION KEYER CIRCUIT 18 Claims, 5 Drawing Figs.

US. Cl. 84/l.26, 84/1.13, 84/124 Int. Cl. G0lh 1/02 Field 01 Search 84/ l .01, 1.13,1.24, 1.26

Relerences Cited UNITED STATES PATENTS 3,180,919 4/1965 Stiefel 84/126 3,247,306 4/1966 Milho 84/1.13 5/1968 Sharp 84/1 .26

3,493,668 2/1970 Bunger Primary Examiner-Milton O. I-lirshfield Assistant Examiner--U. Weldon Attorney-Mueller & Aichele ABSTRACT: A tone generator is coupled to an audio reproducing circuit through a variable threshold gate circuit which operates in response to the actuation of a musical instrument key. Means are provided to apply a threshold voltage signal to the threshold gate circuit to initiate conduction of the gate to allow translation of the signals from the tone generator to the audio reproducing means. A current control device has its load electrodes connected between the key contactor and the input of the variable threshold gate circuit to apply an initial pulse thereto and means are provided to automatically terminate the application of such pulse whether said key contactor remains closed or is opened thereafter to simulate the percussive sound of a piano from an electronic organ AUDIO AMP AND SPEAKER SYSTEM PATENTEDNUV 2 I97l INVENTOR JAMES. S. SOUTHARD SHEET 10F z iwhw m mmxdwmm 024 ATTYS.

PERCUSSION KEYER cmcurr BACKGROUND or THE INVENTION This invention relates generally to electronic musical instruments, and more particularly to keying circuits used for the generation of percussion sounds. This invention can be used to simulate the sounds of any type of percussion instrument but is particularly useful in combination with electronic organs or other keyboard-type electronic musical instruments which are used to simulate the sounds of a piano.

Electronic organs have keyboards which, upon pressing any selected key, cause continuous tone signals to be translated through an audio reproducing system. Upon releasing the key, the tone signal so generated is abruptly terminated and so also is the sound. A crude percussive effect can be obtained by smartly striking a key on the keyboard and releasing it. This produces a short loud tone to be developed at the audio reproducing portion of the organ. However, this tone does not truly simulate the effect of a piano since there is no gradual decay of the tone after the initial striking of the key. For example, when a piano key is held down after being struck, the tone developed within the piano will continue and decrease gradually until dampened by the release of the piano key or actuation of a foot pedal or the like. If neither of these conditions occur, the tone will gradually decay of its own accord.

Some electronic organs use circuits to simulate the effects of a piano keyboard. One such arrangement is where an initial striking of the key causes a loud audio signal to be developed and which decays gradually over a given period of time. However, this is somewhat unnatural in contrast to a piano note in that release of a key does not cause abrupt dampening of the tone as would a conventional piano keyboard. Another circuit arrangement of the prior art for producing percussive effects is to operate the key as a double-pole double-throw switch such that when the key is in the rest position it makes contact to charge a capacitor and upon striking the key the capacitor is discharged to cause energization of the tone generator for a period of time determined by the discharge rate of the capacitor. This method is rather impractical in that it requires a more complicated keyboard structure having contact means for both the rest position and depressed position of the keys. Also this arrangement increases the cost of the organ.

Yet another circuit arrangement of the prior art is where a capacitor is connected in series between the switch formed by the keyboard and a gating circuit such that actuation of the key applies an initial charge through the capacitor to produce an initial pulse at a gating circuit and thereafter the capacitor becomes charged to block further current flow therethrough regardless of the duration the key is depressed. The capacitor used in this arrangement is generally an electrolytic capacitor which is an inherently troublesome circuit component, particularly when a multitude of such capacitors are used with the keys of an electronic organ.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a percussion keying circuit which is efficient and reliable in operation.

Another object of this invention is to provide a percussion keying circuit which more accurately simulates the percussive attack and decay characteristics of a piano sound.

A feature of this invention is the use of a transistor coupled between each key of the keyboard on the organ and a corresponding gating circuit which controls translation of output signals from an audio signal generator through an audioreproducing circuit.

Briefly, the percussion keyer circuits of this invention provide novel circuit arrangements whereby the attack and decay of threshold voltage control signals are obtained by rendering a current control device, such as a transistor, highly conductive for an initial short period of time to form the attack characteristic of the threshold voltage, and upon rendering the current control device nonconductive, or substantially nonconductive, the decay characteristic of the threshold voltage is automatically obtained by circuit components within a threshold keying circuit which couples audio signal information to the input of an audio amplifier and speaker system. In one embodiment of this invention, the attack characteristic of the circuit is obtained by the use of a transistor which has a zener diode effect between two of its electrodes, preferably they being the emitter and base electrodes. This zener diode effect persists only as long as the voltage difference between these electrodes is greater than the zener voltage, but as soon as the voltage decreases to or below the zener voltage the junction of the transistor becomes substantially nonconductive.

In accordance with another embodiment of this invention, simulation of relatively abrupt dampening of the decay characteristics of the threshold voltage is obtained by a discharge current path through the current control device at a junction which forms a forward biased diode either upon release of a key or upon actuation of a selector switch. Therefore, during application of the attack portion of the threshold voltage the current control device conducts heavily in one direction, and upon release of a key, or selector switch, the current control device provides a discharge path through one or more junctions thereof to abruptly dampen the sound produced by the organ to simulate the percussive effect of a piano.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a schematic diagram of one embodiment of a percussion keying circuit of this invention;

FIG. 2 illustrates an alternate embodiment of a percussion keying circuit of this invention;

FIG. 3 illustrates the wave shape obtained by either embodiment of this invention;

FIG. 4 illustrates still another alternate form of this invention; and

FIG. 5 illustrates a wave shape obtained from the circuit of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, there is seen a percussion keying circuit constructed in accordance with the principles of this invention and designated generally by reference numeral 10. Audio signal information of the desired frequency is developed within a signal source 12 and coupled to an audio amplifier and speaker system 14 at one of the plurality of inputs thereof. The audio signal from the signal source 12 is coupled to the audio amplifier and speaker system 14 through a threshold-responsive gating circuit designated generally by reference numeral 16 upon actuation of a key 18. The key 18 is one of a plurality of such keys on a conventional keyboard instrument such as an organ. The key actuates a switch 19 to apply B+ at the terminal 20 through the bus 21 to a load resistor 22.

A current control device, preferably a transistor 24, has the emitter electrode thereof coupled to the load resistor 22 through a series resistor 26. The base electrode of transistor 24 is coupled to ground potential by a capacitor 34. Upon actuation of the key 18, the switch 19 is closed, i.e. it makes contact with the bus bar 21, to apply a positive potential to the emitter electrode of transistor 24 to forward bias the same. Accordingly, transistor 24 is initially rendered highly conductive to apply a full charge across capacitor 32 which is connected in circuit with the collector electrode of transistor 24. After a predetermined period of time, which is relatively short, capacitor 34 is charged through resistor 28 to a voltage sufficient to render transistor 24 nonconductive, thereby abruptly terminating the charge applied to capacitor 32.

The initial conduction of transistor 24 overcomes a threshold-blocking voltage applied to the base of a transistor 38 which forms the threshold-responsive circuit 16. This threshold-blocking voltage is applied through a resistor 40 from a bus bar 42 which may be selectively or permanently connected to the resistor 40, as desired. The thresholdblocking voltage maintains the transistor 38 in a nonconductive state to prevent audio signal information from being coupled from the signal source 12 to the audio amplifier and speaker system 14 until such time as the key 18 is closed. Upon actuation of the key 18, transistor 38 is rendered highly conductive by the threshold-conducting voltage for an initial short period of time because of the initial conduction of the transistor 24. This is best seen by the threshold voltage waveform 41 of FIG. 3. The attack characteristic of the threshold voltage is the high-amplitude portion 41a which occurs at the very beginning of the waveform. After capacitor 34 has become charged sufficient to render transistor 24 nonconductive, the full charge on capacitor 32 is abruptly decreased a slight amount to the point 43 on the waveform 41 as a result of a reduced voltage bias source applied to a bus bar 44 and coupled to the capacitor 32 via resistor 46 and diode 48. The voltage on the bus bar 44 may be in the order of approximately one-third to one-half the voltage applied to terminal 20 at the bus bar 21. At the peak amplitude of the waveform 41 the charge on capacitor 32 is approximately equal to the voltage applied to terminal 20 and substantially more than the bias voltage applied to the bus bar 44. Therefore, upon nonconduction of transistor 24, the diode 48 becomes forward biased rapidly to discharge a portion of the charge on capacitor 32 which produces the attached characteristic of the threshold voltage. When the voltage on capacitor 32 decreases approximately to that of the voltage on bus bar 44, i.e. the point 43 on the waveform 41, the discharge of capacitor 32 will then be through a series-connected resistor 50, the base-emitter junction of the transistor 38 and emitter resistor 52, this discharge rate is less than the first discharge rate provided by the reduced voltage on bus bar 44, and is illustrated by the portion 41b of the waveform 41. From the point 43 of the waveform 41 the discharge of capacitor 32 is automatic and natural, i.e. exponential through the resistive components of the threshold responsive circuit 16. During the rapid decrease, to point 43, and the following gradual decrease along curve 41b, the amount of signal amplitude coupled from the signal source 12 to the audio amplifier and speaker system 14 initially is high and then gradually decreases substantially to simulate the actual attack and decay sound characteristics of a percussion instrument such as a piano, or the like.

Most advantageously, abrupt dampening, or substantially abrupt dampening characteristics of the gradual decay portion 41b of the threshold voltage applied to the base of transistor 38 is accomplished upon release of the key 18, which action removes B+ potential from the load resistor 22. This rapid decay of the threshold voltage value is accomplished by an independent discharge path provided from capacitor 32 through the collector-base junction of transistor 24, the resistor 28, resistor 26 and resistor 22 to ground potential when the key 18 is deactuated. This abrupt dampening is illustrated by the dotted lines starting from the points 45 and may occur at any point in time along the curve 41b.

This unique feature of obtaining a dual function from the transistor 24 greatly simplifies the circuit arrangement of the percussion keying circuit of this invention while also reducing the number of components necessary to obtain a variable threshold voltage potential at the base of transistor 38 corresponding to the natural attack and decay characteristics of a percussion instrument.

To increase the speed of initial response of transistor 38 so that it will become highly conductive upon actuation of the key 18, the current path from resistors 28 and 30 is coupled to the base of transistor 38 via a line 54 and to effectively lengthen the gradual decay of the curve 41b of FIG. 3. How ever, it would be understood that the resistance value of resistor 30 is sufficiently high to provide a relatively slow discharge path for capacitor 34 and to maintain the voltage value when capacitor 32 has completely discharged and the key 18 is still in the closed-circuit condition.

The audio signal information from the signal source 12 is coupled through resistor 56 and developed across the resistor 52 whereupon it is translated through the emitter-collector circuit of the transistor 38. Operating potential is applied to the collector of transistor 38 through a resistor 58. The audio signal information developed across resistor 58 is then coupled through a filter including capacitors 60, 68 and 70 and a pair of resistors 62 and 64 to an input 66, which is one ofa plu rality of inputs, of the audio amplifier and speaker system 14. Capacitors 68 and 70 reduce the effects of undesired highfrequency harmonics of the audio signal information.

As mentioned hereinabove, when the key 18 is released before capacitor 32 has had time to completely discharge, the abrupt dampening of the threshold, as illustrated by one of the dotted lines of the waveform 41, is accomplished by the independent discharge path for capacitor 32. This independent discharge path is provided by a current path through the collector-base junction of the transistor 24.

Referring now to FIG. 2, there is seen an alternate embodiment of this invention. Here a percussion keyer circuit is designated generally by reference numeral 80. Audio signal information from a signal source 82 is coupled to a selected input terminal of an audio amplifier and speaker system 84 through a threshold-responsive circuit designated generally by reference numeral 86. Audio signal information is coupled to the audio amplifier and speaker system 84 upon actuation of a key 88 connected to a switch 91 which applies a B+ potential from a bus bar 89, which is connected to a terminal 90, to a load resistor 92. In this instance, the potential applied through the switch 91 is coupled to the base electrode of a transistor 94 via a resistor 96 and a capacitor 98. Also connected to the base electrode of the transistor 94 is a signal-developing resistor 100. Upon the initial closure of switch 91, transistor 94 becomes highly conductive which, in turn, renders a transistor 102 highly conductive. This is accomplished as a result of a bias resistor 104 connected in series with transistor 94 and coupled between the emitter-base junction of the transistor 102. The high current conducting state of transistor 102 al' lows current to flow through the switch 91, the resistor 96 and a forward-biased diode 106 into a capacitor 108.

The charge on capacitor 108 serves as a variable threshold voltage applied to the base electrode ofa transistor 110 which forms the threshold-responsive circuit 86. The threshold-voltage waveform obtained by the circuit of FIG. 2 is the same as that shown in FIG. 3 and described hereinabove. The charge on capacitor 108 decreases automatically when transistor 102 is rendered nonconductive to provide automatic decrease of the threshold voltage. As was the case with regard to the circuit of FIG 1, here also a reduced bias potential is applied to capacitor 108 through a resistor 112 and a diode 114 which is connected to a bus bar 116. The bus bar 116 may have a voltage approximately equal to one-third to one-half the 13+ voltage applied to terminal 90. Resistor 112 and diode 114 therefore provide an initial abrupt partial discharge of capacitor 108 to simulate the rapid decay of the attack portion of a percussion signal whereupon the voltage across capacitor 108 becomes substantially equal to the bias voltage on bus bar 116. At this point, the capacitor 108 then discharges through a resistor 118 and the base-emitter junction of transistor 110 through a resistor 120 to ground potential.

Operating potential is applied to transistor 110 from a terminal point 122 through a resistor 124. Also, a blocking threshold voltage is applied to the base of the transistor 110 through a resistor 126 from a bus bar 128. However, when a current is applied through a transistor 102 to charge capacitor 108 it greatly overcomes the blocking threshold potential applied to the base of transistor 110 and continues to overcome the blocking bias while capacitor 108 is discharging.

Most advantageously, a third discharge path is provided for capacitor 108 through the collector-base junction of the transistor 102, resistor 104, and a resistor 130 and diode 132. This discharge path will occur only when diode 132 is connected to ground potential through a selector switch 134.

When switch 134 is connected, as shown in FIG. 2, the rapid decay characteristic of the threshold voltage applied to the base of transistor 110 will be provided. However, when switch 134 is actuated to the open-circuit condition, capacitor 108 will continue to discharge only through resistor 118 and baseemitter junction of transistor 110 substantially to simulate the sustained decay of an actual percussion instrument.

Referring now to FIG. 4, there is seen still another alternate embodiment of this invention. Here a percussion keyer circuit is designated generally be reference numeral 150 and includes an audio signal information source 151 which is coupled to a selected input of audio amplifier, not shown, through a threshold-responsive circuit designated generally by reference numeral 152. The audio signal information generated at the source 151 is translated through the threshold-responsive circuit 152 upon actuation of a key 153 which is mechanically coupled to a switch 154 which contacts a bus bar 156. The bus bar 156 receives operating potential from a 13+ terminal 157.

In this particular arrangement, the potential applied through the switch 154 is coupled to the base electrode of a transistor 158 via a capacitor 159. Preferably, the transistor 158 is an NPN-transistor having its emitter electrode-coupled to a charging capacitor 160 through a resistor 161 and its collector electrode connected to the B+ terminal 157. The forward-bias pulse delivered through the capacitor 159 will cause transistor 158 to conduct heavily for an initial short period of time to apply a fast-rising charge on the capacitor 160 corresponding to the initial increasing slope 162 of the characteristic curve 163 shown in FIG. 5. The peak value 162a of the characteristic curve 163 represents the maximum voltage charge delivered to capacitor 160 during the time interval that capacitor 159 is becoming charged, an action which allows transistor 158 to be highly conductive. When capacitor 159 is substantially fully charged, transistor 158 becomes nonconductive whereupon, in accordance with an aspect of the invention, the emitter-base junction of the transistor forms a zener diode of a predetermined threshold voltage V, as indicated in FIG. 5. Therefore, capacitor 160 will rapidly discharge along the slope 164 of the characteristic 163 to the potential V,, this action occuring rapidly to form the attack characteristic of the threshold voltage. After the voltage on capacitor 160 has decreased to or below the threshold-voltage value formed by the zener diode junction of transistor 158, the normal discharge path of capacitor 160 is then through a resistor 166 into the base-emitter junction of a transistor 167 to ground potential through a resistor 168 which forms part of the threshold-responsive circuit 152. Operating potential is applied to the transistor 167 through a resistor 169 and the audio signal applied thereto is current limited through a resistor 170.

A low-value bias potential is applied to the base electrode of transistor 167 via a resistor 171 which is connected to a bias terminal 172. The voltage value of the bias voltage at this ter minal is sufficient to insure that transistor 167 is fully nonconductive during the normal state or condition, but which bias potential is readily overcome by the application of a threshold-voltage value in the form of the characteristic curve 163, FIG. 5. This insures that no signal from the signal source 151 is translated through the transistor 167 until the key 153 is depressed to connect switch 154 to the bus bar 156.

A selector switch 175 is arranged for connection to 3+ potential in one position and to ground potential in the other position selectively to provide alternate discharge paths for capacitor 160. To accomplish this the emitter electrode of transistor 158, at the juncture of resistor 161, is connected to the switch 175 through a resistor 176, a diode 177 and a second resistor 178 which, in turn, is connected to a bus bar 179. Therefore, actuation of switch 175 to either of its two positions will apply either B+ potential or ground potential, whichever the case may be, to the bus bar 179 which, in turn, will control the characteristic operation of all the keys of the keyboard of a musical instrument. The juncture between diode 177 and resistor 178 is connected to ground potential through a resistor 180. It is also connected to the switch 154 so that as long as switch 154 is in the closed position, i.e. in contact with the bus bar 156, 3+ potential will be applied to the cathode of diode 177 to reverse bias the diode to a high-resistant current-blocking condition so that capacitor will not begin to discharge therethrough until the key 153 is released. The base electrode of transistor 158 is connected to ground potential through a resistor 181.

When switch 154 is closed to make contact with the bus bar 156, a positive potential is developed across resistor 180 and coupled therefrom to the base electrode of transistor 158 through the capacitor 159. Preferably, transistor 158 is an NPN-transistor connected as an emitter follower. When 8+ is coupled to the base electrode of transistor 158 through the capacitor 159, the B+ potential at terminal 157 is also applied to capacitor 160 through the resistor 161 as a result of the high-conductive state of transistor 158. The RC time constant formed by resistor 161 and capacitor 160 is relatively short to produce the attack characteristic indicated by the peak portion of the curve 163. The base of transistor 158 rapidly goes to ground potential through resistor 181 when capacitor 159 is fully charged and current flow ceases to pass therethrough.

The discharge path of capacitor 160 is through several discrete circuit elements or paths depending upon the mode of operation of the selector switch 175. With selector switch 175 connected to the 13+ terminal so as to reverse bias the diode 177, the discharge path of capacitor 160 is only through the resistor 166 and the base-emitter junction of transistor 167 and through resistor 168 and R 171, this discharge path forming the portion 185 of the characteristic curve 163 starting at or slightly below the zener voltage vz. However, as mentioned hereinabove, the initial discharge of capacitor 160 is accomplished through the emitter-base junction of transistor 167 as a result of the zener breakover voltage of the diode circuit formed thereby.

In accordance with this embodiment of the invention a key return dampening effect is obtained and illustrated by the dotted characteristic portions 186 and 187 of the characteristic curve 163, thus illustrating a substantially automatic and rapid decay of the threshold voltage at the base electrode of the transistor 167 rapidly to terminate conduction thereof and block any further signal translation therethrough. To obtain this key-return-dampening effect, the selector switch 175 is actuated to the ground potential terminal so that upon open ing the switch 154, by release of the key 153, the charge on capacitor 160 will pass through diode 177 to ground potential both through the resistor 178 and through the resistor 180.

Accordingly, the embodiments of this invention illustrated herein provide novel circuit arrangements whereby the rapid decay of a threshold voltage applied to threshold-responsive transistors 38 and 86 substantially simulates the natural decay obtained from percussion instruments such as pianos, or the like, upon release of the key of a piano or upon manipulation of a foot pedal of the piano.

I claim:

1. A percussion keyer circuit, for selectively connecting an audio signal to audio reproducing means, including the com bination of;

first means having an input for receiving an audio signal and an output adapted to be connected to audio reproducing means and selectively coupling the audio signal to said output in response to the value of a threshold voltage, said first means including means for retaining a threshold voltage applied thereto and automatically causing decay thereof;

a keying circuit selectively actuated to provide a control voltage;

a current control device having load electrodes and a control electrode, said load electrodes being coupled between said keying circuit and said first means for applying said control voltage to said first means to provide said threshold voltage; and

second means connected in circuit with said control electrode of said current control device and responsive to the control voltage to terminate the application of said control voltage to said first means at a predetermined point in time after said keying circuit is actuated to allow the automatic decay of said threshold voltage to become effecuve.

2. The percussion circuit of claim 1 wherein said first means includes a capacitor to be charge by said threshold voltage, and means forming a first discharge path for partially discharging said capacitor at a first discharge rate and means forming a second discharge path for further discharging said capacitor at a second lesser discharge rate.

3. The percussion circuit of claim 2 wherein one of said load electrodes and said control electrode of said current control device form a current discharge path for said capacitor when said keying circuit is deactuated to remove voltage from said current control device to cause a third discharge rate of said capacitor.

4. The percussion keyer circuit of claim 3 wherein said third discharge rate of said capacitor is less than said first discharge rate and greater than said second discharge rate.

5. The percussion keyer circuit of claim 1 including charging circuit means coupled to the control electrode of said current control device for receiving charge in response to the actuation of said keying circuit to render said current control device inoperative when said charging circuit means is charged to a predetermined value.

6. The percussion keyer circuit of claim 1 wherein said current control device is a transistor having a collector electrode connected to said first means and an emitter connected to said keying circuit, and further having a blocking capacitor coupled to the base electrode of said transistor to receive charge in response to the actuation of said keying circuit to render said transistor nonconductive at a predetermined period of time after said keying circuit is actuated.

7. The percussion keyer circuit of claim 6 further including resistance means coupled to the base electrode of said transistor and to said blocking capacitor to cause said transistor to become highly conductive in response to the actuation of said keying circuit, and upon charging of said blocking capacitor to a predetermined value aid transistor is rendered nonconductive, and including means connected in circuit with said blocking capacitor and said resistance means to effect rapid discharge of said capacitor upon deactuation of said keying circuit.

8. The percussion keyer circuit of claim 7 wherein the collector-base junction of said transistor forms a forward-biased diode to provide a discharge path for said means responsive to a threshold voltage when said keying circuit is deactuated.

9. The percussion keyer circuit of claim 1 including a second current control device having load electrodes connected in series with the control electrode of said first current control device and a control electrode coupled to said keying circuit to be rendered conductive in response to actuation of said keying circuit, and a charging circuit coupled between said keying circuit and said control electrode of said second current control device to terminate conduction of said first and second current control devices upon charging of said charging circuit by a predetermined value.

10. The percussion keyer circuit of claim 9 wherein one of said load electrodes and said control electrode of said first current control device form a current discharge path for said first means, and means for effecting conduction of current through said current discharge path during one instance to increase the rate of discharge of said threshold voltage, and for effecting nonconduction of current through said current discharge path during another instance to cause the discharge of said threshold voltage to occur at a normal discharge rate.

11. The percussion keyer circuit of claim 9 wherein said first current control device is a transistor having its collector electrode connected to said first means and its emitter electrode coupled to said keyer circuit, and wherein said second current control device is a transistor having its collector electrode connected to the base electrode of said first transistor and its emitter electrode connected to a reference potential,

the base electrode of said second transistor being coupled to said keying circuit through a capacitor, whereby, actuation of said keying circuit will render said first and second transistors conductive to apply said threshold voltage to said first means and upon charging of said capacitor to a predetermined value said first and second transistors are rendered nonconductive to allow said capacitor to discharge through said first means.

12. The percussion keyer circuit of claim 11 wherein the junction of the emitter and base electrodes of said first transistor form a forward biased diode upon deactuation of said keying circuit, and selector means connected in circuit with the diode so formed for effecting conduction of the said diode during one instance to change the rate of discharge of said capacitor and for effecting nonconduction of the said diode during another instance to allow the normal decay of said capacitor through said means responsive to a threshold voltage.

13. The percussion keyer circuit of claim 12 wherein said means coupled to said diode formed by said first transistor includes a discrete diode connected in series with selector switch means.

14. The percussion keyer circuit of claim 1 wherein one of said load electrodes and said control electrodes of said current control device form a forward biased diode upon deactuation of said keying circuit, and means to effect a current discharge path of said first means through the said diode to change the rate of discharge of said threshold voltage in response thereto.

15. The percussion keyer circuit of claim 1 wherein the said current control device is a transistor with its emitter and collector electrodes coupled between said keying circuit and said first means, and wherein said transistor forms a zcner diode effeet between its base electrode and one of either the emitter or collector electrode to provide a threshold voltage discharge path thus forming an attack characteristic of the threshold voltage applied to said means responsive to a threshold voltage.

16. The percussion keyer circuit of claim 15 wherein the said transistor is an N PN-transistor.

17. The percussion keyer circuit of claim 15 wherein said transistor has its emitter electrode connected to said first means, and its collector electrode connected to said keying circuit, the base electrode of said transistor being connected to said keying circuit through a rapidly chargeable coupling capacitor, a selector switch connectable between a 8+ potential and ground potential, and a diode and resistor circuit coupling the emitter electrode or said transistor to said selector switch.

18. The percussion keyer circuit of claim 17 further including circuit means connecting said selector switch to said keying circuit so as to effect a rapid decay of the threshold voltage value applied to said first means upon the opening of said keying circuit when said selector switch is connected to ground potential.

0 t t t t 

1. A percussion keyer circuit, for selectively connecting an audio signal to audio reproducing means, including the combination of; first means having an input for receiving an audio signal and an output adapted to be connected to audio reproducing means and selectively coupling the audio signal to said output in response to the value of a threshold voltage, said first means including means for retaining a threshold voltage applied thereto and automatically causing decay thereof; a keying circuit selectively actuated to provide a control voltage; a current control device having load electrodes and a control electrode, said load electrodes being coupled between said keying circuit and said first means for applying said control voltage to said first means to provide said threshold voltage; and second means connected in circuit with said control electrode of said current control device and responsive to the control voltage to terminate the application of said control voltage to said first means at a predetermined point in time after said keying circuit is actuated to allow the automatic decay of said threshold voltage to become effective.
 2. The percussion circuit of claim 1 wherein said first means includes a capacitor to be charge by said threshold voltage, and means forming a first discharge path for partially discharging said capacitor at a first discharge rate and means forming a second discharge path for further discharging said capacitor at a second lesser discharge rate.
 3. The percussion circuit of claim 2 wherein one of said load electrodes and said control electrode of said current control device form a current discharge path for said capacitor when said keying circuit is deactuated to remove voltage from said current control device to cause a third discharge rate of said capacitor.
 4. The percussion keyer circuit of claim 3 wherein said third discharge rate of said capacitor is less than said first discharge rate and greater than said second discharge rate.
 5. The percussion keyer circuit of claim 1 including charging circuit means coupled to the control electrode of said current control device for receiving charge in response to the actuation of said keying circuit to render said current control device inoperative when said charging circuit means is charged to a predetermined value.
 6. The percussion keyer circuit of claim 1 wherein said current control device is a transistor having a collector electrode connected to said first means and an emitter connected to said keying circuit, and further having a blocking capacitor coupled to the base electrode of said transistor to receive charge in response to the actuation of said keying circuit to render said transistor nonconductive at a predetermined period of time after said keying circuit is actuated.
 7. The percussion keyer circuit of claim 6 further including resistance means coupled to the base electrode of said transistor and to said blocking capacitor to cause said transistor to become highly conductive in response to the actuation of said keying circuit, and upon charging of said blocking capacitor to a predetermined value aid transistor is rendered nonconductive, and including means connected in circuit with said blocking capacitor and said resistance means to effect rapid discharge of said capacitor upon deactuation of said keying circuit.
 8. The percussion keyer circuit of claim 7 wherein the collector-base junction of said transistor forms a forward-biased diode to provide a discharge path for said means responsive to a threshold voltage when said keying circuit is deactuated.
 9. The percussion keyer circuit of claim 1 including a second current control device having load electrodes connected in series with the control electrode of said first current control device and a control electrode coupled to said keying circuit to be rendered conductive in response to actuation of said keying circuit, and a charging circuit coupled between said keying circuit and said control electrode of said second current control device to terminate conduction of said first and second current control devices upon charging of said charging circuit by a predetermined value.
 10. The percussion keyer circuit of claim 9 wherein one of said load electrodes and said control electrode of said first current control device form a current discharge path for said first means, and means for effecting conduction of current through said current discharge path during one instance to increase the rate of discharge of said threshold voltage, and for effecting nonconduction of current through said current discharge path during another instance to cause the discharge of said threshold voltage to occur at a normal discharge rate.
 11. The percussion keyer circuit of claim 9 wherein said first current control device is a transistor having its collector electrode connected to said first means and its emitter electrode coupled to said keyer circuit, and wherein said second current control device is a transistor having its collector electrode connected to the base electrode of said first transistor and its emitter electrode connected to a reference potential, the base electrode of said second transistor being coupled to said keying circuit through a capacitor, whereby, actuation of said keying circuit will render said first and second transistors conductive to apply said threshold voltage to said first means and upon charging of said capacitor to a predetermined value said first and second transistors are rendered nonconductive to allow said capacitor to discharge through said first means.
 12. The percussion keyer circuit of claim 11 wherein the junction of the emitter and base electrodes of said first transistor form a forward biased diode upon deactuation of said keying circuit, and selector means connected in circuit with the diode so formed for effecting conduction of the said diode during one instance to change the rate of discharge of said capacitor and for effecting nonconduction of the said diode during another instance to allow the normal decay of said capacitor through said means responsive to a threshold voltage.
 13. The percuSsion keyer circuit of claim 12 wherein said means coupled to said diode formed by said first transistor includes a discrete diode connected in series with selector switch means.
 14. The percussion keyer circuit of claim 1 wherein one of said load electrodes and said control electrodes of said current control device form a forward biased diode upon deactuation of said keying circuit, and means to effect a current discharge path of said first means through the said diode to change the rate of discharge of said threshold voltage in response thereto.
 15. The percussion keyer circuit of claim 1 wherein the said current control device is a transistor with its emitter and collector electrodes coupled between said keying circuit and said first means, and wherein said transistor forms a zener diode effect between its base electrode and one of either the emitter or collector electrode to provide a threshold voltage discharge path thus forming an attack characteristic of the threshold voltage applied to said means responsive to a threshold voltage.
 16. The percussion keyer circuit of claim 15 wherein the said transistor is an NPN-transistor.
 17. The percussion keyer circuit of claim 15 wherein said transistor has its emitter electrode connected to said first means, and its collector electrode connected to said keying circuit, the base electrode of said transistor being connected to said keying circuit through a rapidly chargeable coupling capacitor, a selector switch connectable between a B+ potential and ground potential, and a diode and resistor circuit coupling the emitter electrode or said transistor to said selector switch.
 18. The percussion keyer circuit of claim 17 further including circuit means connecting said selector switch to said keying circuit so as to effect a rapid decay of the threshold voltage value applied to said first means upon the opening of said keying circuit when said selector switch is connected to ground potential. 